Recently, with the development of wavelength multiplex communications using fiber amplifiers, it has become the practice to monitor the quantity of light in each wavelength with a photodiode (PD), adjust the quantity of light, and amplify the signal light with an amplifier for the purpose of preserving amplifier characteristics.
Various monitoring processes are known in the art. Since a monitoring device is connected to each optical fiber, the monitoring devices alone are of considerable size.
Therefore, there has been a demand for monitoring devices that are smaller in size and higher in packing density. There has also been a demand for monitoring devices that are capable of monitoring a signal light without significantly attenuating the signal light when a portion of the signal light is extracted.
Heretofore, a technique disclosed in Japanese Laid-Open Patent Publication No. 2001-264594 (see FIGS. 24 and 25), for example, has been proposed. According to the disclosed technique, an optical fiber is placed in a V-shaped groove defined in a glass substrate, and thereafter a parallel groove is defined in the glass substrate obliquely across the optical fiber (the optical axis thereof). A light reflecting base (optical member) is inserted into the parallel groove, and an ultraviolet-curable resin (adhesive) is filled in the gap between the light reflecting base and the groove wall.
A light component (reflected light) of the signal light, which propagates through the optical fiber and which is reflected by the light reflecting base, is extracted out through the cladding. The reflected light is detected by a photodetector device, for example, to monitor the signal light.
If a photodiode (PD) is mounted on an optical fiber, then since a single optical fiber is most usually employed, a metal package type PD is typically mounted in place as shown, for example, in Japanese Laid-Open Patent Publication No. 10-300936 (see FIG. 6), Japanese Laid-Open Patent Publication No. 11-133255 (see FIGS. 1 and 4), and WO 97/06458 (see FIG. 4). This is because the single optical fiber poses smaller spatial limitations and numerous metal package type PDs are available on the market and have proven to be effective in terms of cost and reliability.
However, it has been found difficult to combine multiple optical fibers with metal package type PDs. When multiple optical fibers need to be mounted in a high packing density, with a fiber pitch of 250 μm, then it is necessary to install a photodiode array (PD array) made up from a plurality of bare photodiodes.
It is generally customary to extract electric signals from the PD array using a package comprising a plurality of pins.
In the major conventional designs, electrodes are mounted on an upper surface of a V-groove substrate having an optical fiber array fixed thereto, or on an upper surface of an optical waveguide that is optically coupled to the V-groove substrate, and the package is connected by wires through the electrodes, for example, as disclosed in Japanese Laid-Open Patent Publication No. 7-104146 (see FIG. 2), Japanese Laid-Open Patent Publication No. 2-15203 (see FIG. 6(a)), WO 97/06458 (see FIG. 1B), and Japanese Laid-Open Patent Publication No. 7-159658 (see FIG. 1).
One problem common to both conventional arrangements is that, since the electrodes are mounted on the upper surface of the V-groove substrate or on the upper surface of the optical waveguide, there are limitations imposed when connecting the electrodes to the PD array by wires, as well as on the configuration for mounting the PD array.
For providing a connection to the package by wires, e.g., for effecting wire bonding to the pins of the package, since the wires cannot be too long, wires disposed on the upper surface of the V-groove substrate or on the upper surface of the optical waveguide need to be located closely to such pins. This means that the V-groove substrate or the optical waveguide itself needs to be located closely to the pins, resulting in an undue increase in the width of the V-groove substrate or the optical waveguide, and hence increasing costs.
Furthermore, it is necessary to form the electrodes in relative positional alignment with the V-groove substrate or the optical waveguide. However, such an alignment process is highly time-consuming and complex.
Since the electrodes are mounted on the V-groove substrate after the V-groove has been formed in the substrate, or the electrodes are mounted on an optical waveguide which has already been manufactured, if the mounting of the electrodes suffers a defect, then the V-groove substrate or the optical waveguide are also rendered defective, resulting in a yield reduction.
There is a problem inherent in the V-groove substrate, particularly, when the V-groove substrate is made of glass, which is that the surface of the V-groove substrate is frequently ground, and is not in a state of being a mirror surface. When highly packed wires are mounted on such a rough surface, therefore, they are not preferable in terms of characteristics and reliability. During production, the surface of the substrate is ground before the V-groove is formed therein, so that the grinding machine for forming the V-groove and the machined surface of the substrate are kept precisely parallel to each other. Therefore, the V-groove is formed in the substrate only after the surface of the substrate is ground by a surface grinding machine.
Heretofore, apart from mounting electrodes on the surface of a V-groove substrate or onto an optical waveguide, there have been disclosed many examples in which a PD array itself is placed in contact with optical fibers, as shown, for example, in WO 97/06458 (see FIG. 21), Japanese Laid-Open Patent Publication No. 63-191111 (see FIGS. 1 through 3), Japanese Laid-Open Patent Publication No. 2000-249874 (see FIGS. 1 through 4), and Japanese Laid-Open Patent Publication No. 3-103804 (see FIGS. 7(A) and 7(B)). Most of these examples disclose nothing about wiring connections to be made after the installation of the PD array. For making the connection to the package by wires, another wiring board may be placed between the PD array and the optical fiber array, and the PD array may be connected through the other wiring board, or wire bonding may be performed directly from the PD array.
When the PD array is connected by wires through another wiring board, then the other wiring board needs to be newly installed (positioned and bonded) after the PD array has been installed, and then wiring is performed to complete the connection. If direct wire bonding is made from the PD array, then the required wires tend to be quite long and poor in reliability, and further the wire bonding process is highly difficult to perform because complex wiring is made directly from the PD array, which contains highly packed PDs.
For positioning the PD array, it is the general practice to align the PD array while confirming output currents therefrom. According to both the above processes, the positioning process is also highly difficult to perform, as a probe must be applied directly to the PD array during the positioning stage.
Another example, which discloses a mounting process including connections to the PD array by wires, is described in Japanese Laid-Open Patent Publication No. 2002-182052 (see FIG. 2). According to the disclosed example, a PD array is mounted on an auxiliary mount. Specifically, a portion of the PD array is mounted on the auxiliary mount, and another portion (active layer) of the PD array is positioned within the optical path of reflected light using an adhesive interposed therebetween. That is, since any obstacles present in the optical path, which extends to the active layer of the PD array, would prevent the PD array from functioning as a monitor, portions of the PD array other than the active layer are mounted on the auxiliary mount.
With this arrangement, however, when stresses are developed due to curing shrinkage or thermal expansion variations of the adhesive, strain stresses are applied to the PD array since there is a moment fulcrum on the PD array itself. The stresses applied to the PD array are unfavorable because they tend to adversely affect the characteristics thereof, e.g., producing increases in dark current.
The PD array is mounted on the auxiliary mount, and interconnections therefor are likewise formed on the auxiliary mount. If the auxiliary mount suffers warpage or the like, then the assembly suffers various problems such as unreliable conduction or generation of dark current, etc. In view of these shortcomings, the auxiliary mount is required to have a thickness of 0.5 mm or greater. According to the conventional arrangement, therefore, the distance between the optical fiber array and the PD array cannot be reduced to less than 0.5 mm, increasing the possibility of loss of reflected light and crosstalk. Moreover, the auxiliary mount poses limitations on efforts to make the monitor smaller in size and increase monitor performance.
The present invention has been made in view of the above drawbacks. It is an object of the present invention to provide an optical device which makes it possible to position the detecting surface of a photodetector device closely to the surface of an optical transmitting means (e.g., the surface of the cladding of an optical fiber), thus effectively reducing size, increasing detection sensitivity and reducing crosstalk, together with a method of manufacturing such an optical device.